For the current manufacture of industrial or commercial solar cells, the most important task is a reduction of the manufacture cost. In general, the method of manufacturing solar cells through the following steps is widely used.
First, an n-type silicon substrate is provided. A single crystal silicon ingot is prepared by Czochralski (CZ) method, or a polycrystalline silicon ingot prepared by the cast method. The ingot is sliced by the multi-wire saw method, obtaining an n-type silicon substrate. The substrate is then immersed in alkaline solution for removing any damages on the substrate surface caused by slicing. Both the front (light-receiving) and back surfaces of the substrate are provided with a micro-texture structure having a maximum height of the order of 10 μm. Subsequently, dopants are thermally diffused into the substrate by a variety of methods to form p- and n-type diffusion layers. Further, TiO2 or SiN is deposited on the light-receiving surface to a thickness of about 70 nm, for example, to form a passivation film having antireflection ability. Next, a silver-based paste is printed on both the surfaces and fired to form electrodes. The electrode on the light-receiving surface is of comb-shape having a width of about 100 to 200 for example.
Although this method consists of only the necessary minimum number of steps to construct the device, it is regarded superior because of the concomitant effects of enhancing solar cell performance such as energy conversion efficiency. For example, the thermal diffusion of dopant in forming a diffusion layer in a substrate functions to improve the diffusion length of minority carriers in bulk due to the gettering effect. Moreover, the antireflection film not only has the optical effect or reflectivity reducing effect, but also functions to reduce the recombination rate of carriers generated in proximity to the silicon surface. By virtue of the necessary minimum number of steps and several useful effects, industrial or commercial solar cells are now manufactured at lower cost than before.
Meanwhile, the means for forming the diffusion layer includes gas phase diffusion and coating/diffusion. The gas phase diffusion method generally uses POCl3 as the n-type impurity source and BBr3 as the p-type impurity source.
For the coating/diffusion method, spin coating and screen printing are typically employed. Spin coating is performed by dropping a coating fluid containing p- or n-type impurity source to the substrate surface, and spinning the substrate at a high speed, whereby a coating having a uniform thickness can be formed on the substrate surface. The substrate is then heat treated to form a p- or n-type diffusion layer. In the case of screen printing, a p- or n-type diffusion layer can be similarly formed.
In order for the coating/diffusion method to form a diffusion layer having a uniform impurity concentration, not only the coating fluid containing impurity source must be homogeneous, but also the coating fluid must be coated on the semiconductor substrate to a uniform composition and a sufficient thickness. One known coating fluid for impurity diffusion is the coating source described in JP-B S62-027529, for example.